Time base circuits have been utilized for a number of years as the core of most electronic systems. These circuits can either be utilized to provide clock circuits or to provide a pulse at periodic intervals, among other applications. One type of circuit that utilizes the periodic interval type of time base is a "Timeout" circuit. Timeout circuits are typically preset to a predetermined time in an internal counter circuit, and an internal oscillator counts down from the preset time reference to zero time when an output pulse is provided. If the preset pulse is again received, the circuit then repeats this operation. At the heart of these circuits is a clock that determines the rate at which the counter operates.
One application for a Timeout circuit is a software protection key system. In this type of system, a supplier presets the Timeout circuit for a predetermined time. The Timeout circuit thus has a predetermined countdown duration and a Timeout pulse is produced after this duration. This Timeout pulse provides a signal to the software key that is utilized to invalidate the key and renders it useless. For example, a supplier or provider of the software key may wish to allow an individual access to a certain software system for a predetermined number of days. Once preset, the Timeout circuit allows the key to operate for this predetermined number of days, after which operation of the software key is inhibited.
One disadvantage to a Timeout circuit is the inherent accuracy of the circuits utilized to realize the Timeout circuit. Typically, some type of analog time base oscillator is utilized, which output is then divided down to give a relatively long time base. The dividing circuits are typically digital circuits which, of course, are very accurate. However, by comparison, the analog time base has inherent inaccuracies due to fabrication tolerances, power supply levels, etc., which directly affect the operation of the circuit. Typically, the fabrication tolerances in an analog oscillator are accounted for by trimming either the value of a resistor or the value of a capacitor that is associated with the timing components in the oscillator. Trimming has some inherent disadvantages in that conventional techniques are expensive to implement and require relatively sophisticated test procedures and/or circuit design techniques. Additionally, these trimming techniques are normally performed during test on an ideal power supply, which has a voltage level that may differ from that actually used in the Timeout circuit during operation in a specific application.
In view of the above disadvantages, there exists a need for a time base circuit which can be utilized as a Timeout circuit, which does not require expensive trimming techniques, and which provides a frequency that does not vary appreciably as a function of the power supply level.